Many different types of industrial dryers exist or have been suggested. For drying of a specific type of material a single or a few types of dryers will often be more suitable than other types.
For drying semi-wet materials, which can be dispersed as particles in a gas stream and require only a relative short residence time in the stream of drying gas, a so-called pneumatic dryer or flash dryer is suitable. In a flash dryer the product to be dried is pneumatically transported through a flash pipe by the drying gas. The flash dryer is used when the physical properties of a wet material allow it to be dried in a matter of seconds by evenly dispersing it into a stream of heated gas, In the present specification and claims the term gas is used in a broad sense, comprising also atmospheric air, and in fact the drying gas will most often be heated atmospheric air.
The extremely short retention time makes a pneumatic dryer ideal for products having a small particle size with surface moisture, or where any required diffusion to the surface occurs rapidly. Relatively high drying temperatures can be used as the immediate evaporation of the surface moisture cools the particle surface and the drying gas, whereby drying can be accomplished without an appreciable increase of the product temperature. Therefore pneumatic drying can be used also for heat sensitive products.
However, for some products suitable for flash drying the residual moisture level of the product when leaving the dryer is to high. This applies inter alia for some types of polyvinyl chloride (PVC). This necessitates an after-drying of the product to bring the residual moisture level from, for example, approximately 2-5% by weight to, for example, approximately 0.1% by weight. An after-drying of products from other types of dryers such as spray dryer and vortex dryers is also often needed. Such an after-drying is conventionally performed in different types of dryers, e.g. fluid bed dryers, ring dryers or cyclone dryers.
A cyclone dryer is known from U.S. Pat. Nos. 4,089,119 and 5,333,392, incorporated herein by reference.
This dryer consists of a cylindrical vertical vessel having a tangential inlet opening in the bottom part and a tangential outlet to obtain a swirling flow of drying gas and particles to be dried within the vessel. To increase the retention time annular baffle plates are arranged in the interior of the vessel. Each baffle plate has a central opening towards which the plate is preferably inclined. Thereby particulate material, which settles on the upper surface of the baffle plates slides towards the opening to meet the rising stream of drying gas.
Such a dryer can be connected in series after a flash dryer. The advantage of such system is inter alia that only one drying gas is needed when using the same gas for flash drying and for after-drying in the cyclone dryer.
However, even if many baffle plates are used in such a prior art cyclone dryer the residence time for particulate material to be dried is shorter than desired for certain products. When relative long residence times are needed this can be obtained only by minimizing the product rate resulting in an increase of the outlet temperature. As a result drying economy is reduced and also the risk of heat damaging the product increases, and further calling for the need of an external cooling device. Thereby investment cost increases and the operation becomes more complicated and the advantage of using only one gas source is thus obviated.
Additionally, the prior art cyclone dryers of the type described above do not ensure optimal conditions of contact between drying gas and particles to be dried at all stages, since contact conditions between the drying air and particles forming a layer on the annular baffle plates are not optimal and consequently the prolonged residence time for the particles is not reflected in a corresponding water evaporation.